Tuning Hierarchical Cluster Assembly in Pulsed Laser Deposition of Al-doped ZnO
Paolo Gondoni1, Valeria Russo1, Carlo E. Bottani1,2, Andrea Li Bassi1,2 and Carlo S. Casari1,2
1Dipartimento di Energia and NEMAS – Center for NanoEngineered Materials and Surfaces, Politecnico di Milano, via Ponzio 34/3, I-20133 Milano, Italy 2Center for Nano Science and Technology @Polimi, Istituto Italiano di Tecnologia, Via Pascoli 70/3, I-20133 Milano, Italy ABSTRACT The synthesis of hierarchically assembled Al-doped ZnO layers by Pulsed Laser Deposition (PLD) at room temperature was investigated. PLD was performed in a background pressure of 100 Pa O2 to deposit clusters in a low energy regime and obtain nano- and mesostructures resulting from a hierarchical assembly of nanoclusters. We here analyzed the effects of varying the gas flow rate on mesoscale morphology, mass density and optical properties. The variation of the target-to-substrate distance was also investigated, identifying its effects on mass density and film morphology. The optimization of optical properties in terms of transparency and light scattering capability is of potential interest for photovoltaic applications. INTRODUCTION Hierarchically structured materials have recently been intensively studied due to their promising functional properties for employment in several fields, from energy harvesting to gas sensing [1,2]. The possibility to employ hierarchical nanostructures as photoanodes in dye-sensitized solar cells has been demonstrated for various metal oxides[3-5]. Furthermore, the benefits in device efficiency arising from the control of light trapping phenomena at the nano- and meso- scale have also been investigated [6,7]. We have recently demonstrated the growth of transparent conducting Al-doped ZnO (AZO) grown by Pulsed Laser Deposition (PLD) in oxygen atmosphere at room temperature [8] and the synthesis of hierarchically assembled AZO films with optimized light scattering properties [9]. In those works we studied the effects of the background oxygen pressure during the PLD process, and identified the optimal deposition conditions to obtain compact transparent conductors and mesoporous photon scatterers. In particular, we demonstrated how an increase in the O2 pressure allowed to control clustering phenomena in the ablation plume, decrease the kinetic energy of the ablated species and lead to the growth of forest-like structures. We identified a background pressure of 100 Pa O2 as a threshold for the synthesis of mesoporous hierarchically assembled structures. In the present work, we investigate different paths to achieve a better control of mesoscale morphology and density of hierarchically assembled AZO structures, analyzing the effects of gas flow and target-to-substrate distance on film morphology, structure and optical properties. In order to test the compatibility with organic substrates of interest for application to flexible optoelectronics and energy conversion devices, depositions were performed at room temperature also on polymer substrates.
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EXPERIMENT The ablation of an Al2O3(2% wt.):ZnO target was performed by a ns-pulsed laser (Nd:YAG 4th harmonic, λ=266 nm, pulse repetition rate 10 Hz, pulse duration 6-8 ns) with an energy density of about 1 J cm-2 on the target. The target-to-substrate distance was varied between 40 mm and 70 mm. Soda-lime glass, Si(100) and Ethylene Tetrafluoroethylene (ETFE) were used as substrates. After evacuating the deposition chamber down to 10-3 Pa, the depositions were performed in a background O2 pressure of 100 Pa, with different gas flow rates and pumping conditions. The O2 flow rate was varied from zero (static, without pumping) to 450 sccm (maximum pumping speed) with a mass flow controller (mks 2179a), by adjusting the conductance of the pumping system (Varian Triscroll 600 dry scroll pump) in order to maintain the same total pressure with different flow rates. The introduction of the gas was in a direction perpendicular to the ablation plume. The number of laser shots per deposited sample was fixed (18000). Film morphology was characterized by Scanning Electron Microscopy (Zeiss SUPRA 40 Field Emission SEM), deposition rates were measured with an Infcon XTC/2 Quartz Crystal Microbalance. Mass densities were calculated from the combination of deposition rate measurements and thickness measurements by cross-sectional SEM. Optical transmittance spectra (total and diffuse) were acquired with a UV–vis–NIR PerkinElmer Lambda 1050 spectrophotometer with a 150 mm diameter integrating sphere. Diffuse transmittance was measured by letting the unscattered light out of the sphere through a slit, and haze was calculated as the ratio of diffuse to total transmittance. RESULTS AND DISCUSSION During the PLD process, the ablated species expand towards the substrate in the form of a plume, whose time-integrated visible length [10] and dimensions can be controlled by varying the background pressure [11]. A pressure increase is known to cause spatial confinement of the plume and clustering phenomena in the gas phase, leading to the deposition of aggregates with low kinetic energy [12]. If the pressure in the chamber is maintained constant, the growth conditions can be varied by varying the gas flow rate or the target-to-substrate distance (dts), as discussed in the present work. Control of morphology Figure 1 reports cross-sectional SEM images of films grown in different flow conditions (a-d) and with different dts (e-f). For figures 1.a – 1.d, the variation of the O2 flow rate was performed in the 0-450 sccm range, with a total pressure of 100 Pa O2 and a dts of 50 mm. The growth in static conditions occurs in a very disordered way, as the film in figure 1.a is constituted by large cauliflower-like aggregates assembled without a preferential direction. The average thickness of films grown in these conditions is very high, and its variation over the sample surface is significant. Deposition in dynamic conditions (with 100 sccm oxygen flowing through the chamber) results in sensibly less thick films (see fig.1b), with increased homogeneity over the material surface. As the gas flow is increased to 450 sccm, film thickness is further reduced and more uniform; the presence of a preferential growth direction (orthogonal to the substrate surface) is observed (fig. 1.c). The nanosized clusters which form the hierarchical assembly can be seen in the higher magnification SEM image reported in fig. 1.d. A hypothesis
to account for this morphology difference can be formulated if we consider that a larger gas flow, and an accordingly higher pumping speed, are able to selectively allow the deposition of more energetic clusters, thus leading to an anisotropic growth. Large, porous cauliflower-like aggregates are featured only in films grown in low flow or static regime.
Figure 1 (a-d) Cross-sectional SEM images of samples grown at 100 Pa O2 with a target-to-substrate distance of 50 mm in different flow conditions: static (a), 100 sccm (b), 450 sccm (c-d). (e-f) Cross-sectional SEM images of samples grown at 100 Pa with a 240 sccm flow rate, with a target-to-substrate distance of 40 mm (e) and 60 mm (f). If the laser fluence, gas pressure and flow conditions are left unchanged, the ablation plume maintains the same shape and length. In this case, film growth can be controlled by varying dts. The effects of a variation of dts on film morphology can be seen in fig. 1.e – 1.f, where cross-sectional SEM images of films grown at 240 sccm with increasing dts are reported. The variation of dts was performed in a range close to the visible edge of the ablation plume. If the substrate is kept closer to the target (fig. 1.e, dts = 40 mm) the material appears more compact and amorphous-like, with no preferential growth direction. An increase in dts up to 60 mm increases the porosity of the film, as can be seen in fig. 1.f. Control of mass density Figure 2 reports the trend in mass density as a function of dts and oxygen flow rate.
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Figure 2 Calculated mass density as a function of dts for materials deposited in different flow conditions. For all the investigated values of oxygen flow, a decreasing trend in mass density with dts is found. The values for films grown at 100 sccm and 240 sccm are of the order of few grams per cubic centimeter, ranging from values close to bulk ZnO (both are over 3 g/cm3 when the substrate and target are closest; the density of bulk ZnO is 5.6 g/cm3) to about 1 g/cm3 as the distance is increased from 40 mm to 70 mm. The films grown in static conditions show mass densities almost one order of magnitude lower, decreasing from 0.4 to 0.1 g/cm3. This trend can be explained by considering that the ablated species cover a longer distance before hitting the substrate, and therefore experience a higher number of collisions with the background gas: this leads to a decrease in deposition kinetic energy, causing the formation of more disordered, porous assemblies.
Control of light scattering properties The results of the optical characterization of materials grown with a target-to-substrate distance of 50 mm are presented in figure 3, which shows the spectral variation of haze (i.e. the ratio of diffuse to total transmittance) in the range from 400 nm to 700 nm, as a function of gas flow rate.
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Figure 3 Haze (calculated as the ratio of diffuse transmittance to total transmittance) in the visible (400 nm – 700 nm) range, for films grown in different flow conditions with dts = 50 mm. The haze of films grown in a static atmosphere is above 90% over the whole visible range, due to the high diffuse transmittance at any wavelength. Films grown at a higher flow rate exhibit a decreasing spectral trend in haze, with average values decreasing with increasing flow rate. This can be related to the differences in morphologies (see fig. 1): films grown in a static atmosphere contain aggregates randomly assembled in structures whose size ranges from the nano- to the microscale, whereas films grown at higher flow rates are increasingly compact, with structures less varying in characteristic size and more anisotropic. Light scattering is expected to be maximum at wavelengths closer to the size of structures featured in the material. The absolute values of haze increase significantly with film thickness, as opposed to total transmittance: total transmittance (not shown) varies from 60% in static conditions to 90% at 450 sccm. It is important to notice how optical properties can be controlled by varying the film thickness (i.e. deposition time). In this work we decided to maintain a constant deposition time to highlight the effects of gas flow and dts, but the synthesis of hierarchically structured AZO with controlled haze and constant thickness has been previously demonstrated [9]. As a final remark, we also point out that we successfully managed to deposit the films on plastic (Ethylene-Tetrafluoroethylene, ETFE) substrates, with uniform adhesion and sufficient mechanical stability. A top view of a sample grown with a flow rate of 100 sccm and a dts of 50 mm is reported, as an example, in figure 4. The importance of depositing on ETFE substrates can be significant in flexible optoelectronic and photovoltaic devices which require light trapping properties.
Figure 4 SEM image of a sample grown with an oxygen flow rate of 100 sccm and a dts of 50 mm on Ethylene-Tetrafluoroethylene (ETFE) substrate.
CONCLUSIONS Hierarchically structured AZO films were grown by PLD at room temperature in an oxygen atmosphere with a pressure of 100 Pa, on soda-lime glass, Si(100) and ETFE plastic substrates. The effects of varying the gas flow rate were discussed, highlighting a gradual transition from thick, porous, disordered films to thinner materials with an anisotropic growth direction as the oxygen flow rate was increased from zero to 450 sccm. Increasing the target-to-substrate distance caused a decrease in film density for every gas flow condition. The obtained mass densities ranged from 0.1 g/cm3 for low flow and high distance, up to 3.7 g/cm3 for high flow and low distance, consistently with what observed for morphology. Finally, the effects of gas flow rate on light scattering properties were investigated, showing a variation of haze from 10% to 90% by controlling morphology, aggregates size and film thickness. Such control, together with room temperature deposition, can be exploited to produce transparent electrodes with ad-hoc light scattering capability for potential applications in flexible solar devices. REFERENCES
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